1. Field of the Invention
The present invention relates to the field of display rate-conversion and, more particularly, to a system and techniques for detecting repeated fields in video data.
2. Related Art
With the increasing deployment of digital video systems, there is a growing demand for the conversion of film material to video. The display in 60 fields/second video format of moving pictures which originated on 24 frames/second film presents a problem of adjusting the picture display rate. This is usually accomplished by a technique known as 3:2 pulldown, described below, in which video fields are created from the appropriate lines of the most recent film frame.
Denote by A, B, C, and D, four consecutive frames from a film. Each frame consists of a certain number of lines which are numbered starting at zero. The top (even) and the bottom (odd) field of a frame consists of the even and odd numbered lines, respectively. Denote by A0, B0, C0 , D0 and A1, B1, C1, D1 the top and the bottom fields of frames A, B, C, and D, respectively. Thus, in terms of its fields, frame A consists of two fields, A0 and A1 (denoted by A0/A1). Application of 3:2 pulldown to the sequence A, B, C, D produces A0/A1, B0/B1, B0/C1, C0/D1, D0/D1.
The 3:2 pulldown is normally used in the film-to-tape transfer process, i.e., while transferring 24 frames/second film material to 60 fields/second video tape. The process ordinarily results in a periodic pattern of repeated fields, which are identical to the previous field of the same parity except for noise introduced in the film-to-tape transfer process. In the MPEG-2 video compression standard, a mechanism is available for signalling that such a repeated field exists in the source. When an encoder detects that a particular field is repeated it sends information used to reconstruct that field only once, so bits which would otherwise be used to transmit redundant information can be used to improve the overall reconstructed quality of the sequence. Moreover, after the 3:2 pulldown has been inverted each frame that the encoder transmits contains two fields from a single time instant; the frame is non-interlaced. Encoding and pre-processing (e.g., noise reduction filtering) may be more effective on the recovered non-interlaced source than on the interlaced version. The MPEG-2 standard does not specify how to determine whether a field is repeated.
Thus a general problem in the field of video rate conversions is how to best detect when a field is "identical" to the next field of the same parity, within the limits of the film-to-tape transfer process.